1. Field of the Invention
The present invention relates to a process for purifying vinylphenol polymers, and, more particularly, to a process for purifying vinylphenol polymers for obtaining purified vinylphenol polymers having a superior light transmittance and an extremely low metal impurity content required for photoresist materials which are used in fine works for the manufacture of super integrated semiconductor elements.
2. Description of the Prior Art
The degree of integration in semiconductor elements is infinitely going up so that the lithography technology for processing high integrated simiconductors must endlessly cope with super miniaturization requirements. A specific requirement in the current pattern-forming technology is capability of producing miniature patterns of the order of 0.5 .mu.m or less. In recent years, deep ultraviolet lights having a wavelength shorter than ultraviolet lights conventionally used for exposing photoresists are regarded to be a more favorable exposure means in order to promote the pattern resolving capability. The KrF excimer laser having a wavelength of 248 nm is supposed to be applied to the generation of deep ultraviolet lights. Several types of photoresists to be exposed to the KrF excimer laser lights have been developed. They are a two component type comprising a photosensitive material and a base resin; a three component type comprising a photosensitive material, a dissolution controlling agent, and a base resin; a three component type comprising a photosensitive material, a cross-linking agent, and a base resin; and the like. Here, the base resin means a component forming films when the photoresist is coated onto silicon wafer surfaces and playing a major role in resisting the etching when the silicon wafer substrates are etched after exposure to lights and development. Novolak-type resins have been widely used as base resins for positive-type resists containing a quinone azide photosensitive agent to be exposed to ultraviolet lights. Since novolak-type resins have a strong absorbancy for deep ultraviolet lights in the neighborhood of 248 nm, preventing the short wavelength lights from reaching the bottom of the resist film, they cannot provide a sufficient resolve of minute patterns when applied to a photoresist material to be exposed to excimer laser lights. Because of this, a number of materials have been studied with respect to their applicability to a base resin for short wavelength photoresists. These studies revealed the extreme usefulness of vinylphenol-type polymers, such as poly-p-vinylphenol, etc., as resins satisfying the requirements not only of the light transmittance but also of the solubility in alkaline solution, the plasma resistance, and the like.
Even though vinylphenol polymers inherently exhibit excellent transmittance for deep ultraviolet lights in the neighborhood of 248 nm, such vinylphenol polymers with excellent transmittance cannot be obtained unless special precautions are taken in the manufacturing process, such as sufficient purification of raw monomers, adoption of a low temperature procedure avoiding heating in the polymerization of monomers and isolation of the polymer product from the polymerization reaction mixture, and the like. However, applying these procedures to an industrial production is very difficult and uneconomical because of the unstable nature of vinylphenol monomers which prevents their easy handling and storing.
In order to improve a low transmittance of conventional vinylphenol polymers manufactured from crude vinylphenol monomer as a starting raw material, a process has been proposed, in which the vinylphenol polymers are contacted with hydrogen in the presence of a Group VIII metal catalyst (Japanese Patent Laid-open No. Hei 1 (1989)-103604). The process has made it possible to easily manufacture vinylphenol polymers having excellent transmittance for deep ultraviolet lights in the neighborhood of 248 nm in an industrial scale.
One of the important properties other than light transmittance required for photoresists exposed to short wavelength lights from the excimer laser or the like is their low metal content. Metals contained in photoresist materials function as pollutants which inhibit the functions of semiconductor elements by attaching to silicon wafer substrates when the plasma etching is performed after the pattern formation by exposure and development. Therefore, minimizing the metal impurity content in a base resin which is the major component constructing photoresists is demanded as a matter of course. Metal contents allowable in base resins have been markedly lowered along with the progress of miniaturization of pattern sizes; i.e., an extremely low level metal content, for example, several tens ppb to several ppb, is required for each metal, e.g., sodium, iron, etc., for the manufacture of semiconductor elements with a pattern size of 0.5 .mu.m or less. However, the content of each metal, e.g., sodium, iron, etc., in vinylphenol polymers manufactured by conventional processes generally exceeds 100 ppb. There have been no technology known in the art to remove these metals from vinylphenol polymers. Thus, no vinylphenol polymer with a high transmittance for deep ultraviolet lights and a low metal content has been manufactured heretofore.